In a motor driving system including a three-phase inverter circuit, which outputs motor driving currents supplied to a motor for driving a vehicle, an electronic control unit (ECU) is configured conventionally to feedback-control motor driving currents to drive the vehicle properly. Specifically, the ECU detects a current flowing in each phase of the three-phase inverter circuit by using a current sensor and determines a feedback control value by comparing a current detection value with a current command value for the motor.
In a case that the current detection value of the current sensor includes an offset, the feedback control value also includes the offset. As a result, the motor driving current of each phase fluctuates and causes torque variation, which results in poor performance in controlling the vehicle.
To counter this problem, patent document JP 2006-258745 A (US 2009/0309528 A1) proposes a technology to learn a quantity an offset (offset value) of the current sensor and correct the output of the current sensor (offset correction). Specifically, in a driving system having plural sets of motors and three-phase inverter circuits as well as a motor control ECU, in which a CPU is provided for driving each motor, the offset of the current sensor is corrected on condition that all of the three-phase inverter circuits are in non-conductive states (off-states) and no power is supplied to the motor. This condition corresponds to a state, in which the vehicle is in a travel stop state and in a regeneration stop state and a shift position of a transmission is neutral.
In the motor driving system disclosed in the above-described patent document, the CPU is reset occasionally because of an uncontrollable operation of the CPU in a course of traveling of the vehicle (shift position is in the drive range, that is, D-position). The offset value, which has been learned and stored in a volatile memory, is lost when the CPU is reset. The offset value need be learned again for controlling the vehicle with high accuracy. While the vehicle is traveling, the shift position is in the D-position, the condition for the offset correction is not satisfied and hence the offset of the current sensor cannot be corrected until the shift position is changed to the neutral range, that is, N-position. As a result, the feedback control is continued without the offset correction until the shift position is changed to the N-position. This continued feedback control causes poor control performance.